How can one pay Zakat for past years in which it was not paid, and is it permissible to deduct the Zakat of one year from the amount designated for the following year?

If the wealth reaches the nisab and a year has passed over it, then zakat is obligatory upon it. She must pay zakat for the previous years by determining the value of 85 grams of gold for each year separately, then giving out 2.5% of the wealth if it equals or exceeds this value. The amount obligatory to be given out as zakat for a year is deducted from the wealth. If the wealth, after deduction, still reaches the nisab, zakat is paid on it in the same manner for the subsequent years.